Auto merge of rust-lang#130628 - workingjubilee:clean-up-result-ffi-guarantees, r=RalfJung

Finish stabilization of `result_ffi_guarantees`

The internal linting has been changed, so all that is left is making sure we stabilize what we want to stabilize.

Author: bors

compiler/rustc_feature/src/accepted.rs

@@ -353,6 +353,9 @@ declare_features! (
     (accepted, repr_packed, "1.33.0", Some(33158)),
     /// Allows `#[repr(transparent)]` attribute on newtype structs.
     (accepted, repr_transparent, "1.28.0", Some(43036)),
+    /// Allows enums like Result<T, E> to be used across FFI, if T's niche value can
+    /// be used to describe E or vice-versa.
+    (accepted, result_ffi_guarantees, "CURRENT_RUSTC_VERSION", Some(110503)),
     /// Allows return-position `impl Trait` in traits.
     (accepted, return_position_impl_trait_in_trait, "1.75.0", Some(91611)),
     /// Allows code like `let x: &'static u32 = &42` to work (RFC 1414).

compiler/rustc_feature/src/unstable.rs

@@ -580,9 +580,6 @@ declare_features! (
     (incomplete, repr128, "1.16.0", Some(56071)),
     /// Allows `repr(simd)` and importing the various simd intrinsics.
     (unstable, repr_simd, "1.4.0", Some(27731)),
-    /// Allows enums like Result<T, E> to be used across FFI, if T's niche value can
-    /// be used to describe E or vise-versa.
-    (unstable, result_ffi_guarantees, "1.80.0", Some(110503)),
     /// Allows bounding the return type of AFIT/RPITIT.
     (unstable, return_type_notation, "1.70.0", Some(109417)),
     /// Allows `extern "rust-cold"`.

compiler/rustc_lint/src/types.rs

@@ -730,7 +730,6 @@ fn is_niche_optimization_candidate<'tcx>(
 /// can, return the type that `ty` can be safely converted to, otherwise return `None`.
 /// Currently restricted to function pointers, boxes, references, `core::num::NonZero`,
 /// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
-/// FIXME: This duplicates code in codegen.
 pub(crate) fn repr_nullable_ptr<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -743,10 +742,6 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
             [var_one, var_two] => match (&var_one.fields.raw[..], &var_two.fields.raw[..]) {
                 ([], [field]) | ([field], []) => field.ty(tcx, args),
                 ([field1], [field2]) => {
-                    if !tcx.features().result_ffi_guarantees {
-                        return None;
-                    }
-
                     let ty1 = field1.ty(tcx, args);
                     let ty2 = field2.ty(tcx, args);